1. Field of the Invention
The present invention relates to amorphous silicon germanium (hereinafter referred to as a - SiGe) films used for semiconductor devices such as a solar cell, a light sensor, a thin film transistor and other types of semiconductor devices using the a - SiGe films.
2. Description of the Prior Art
It is necessary that a photovoltaic device, such as a solar cell or a light sensor semiconductor device can sufficiently absorb incident light so as to improve light sensitivity over a desired wavelength sensitivity region.
Particularly, it is important that the solar cell has absorption characteristics covering a wavelength included in incident light. In recent years, an amorphous semiconductor material has been commonly used as a semiconductor material for the solar cell. The reason for this is that the absorption characteristics of the amorphous semiconductor material can be relatively easily controlled.
The most typical example of the amorphous semiconductor material is an amorphous silicon (hereinafter referred to as a - Si) film.
FIG. 7 is a device structure of a general photovoltaic device composed of an a - Si film. In FIG. 7, a transparent substrate 1 is a supporting member of the photovoltaic device, and is composed of glass or the like. A transparent conductive film 2 for introducing light incident through the substrate 1 into a power generating portion, as described later, serving as one electrode of the photovoltaic device is formed on the transparent substrate 1. The transparent conductive film 2 is composed of ITO, SnO.sub.2 or the like.
A power generating portion (SC) is formed on the transparent conductive film 2. The power generating portion (SC) has a laminated body of a p-type a - Si layer (p), an intrinsic a - Si layer (i) and an n-type a - Si layer (n). A metal electrode portion 3 serving as the other electrode of the photovoltaic device is formed on the power generating portion (SC). The metal electrode portion 3 is composed of aluminum, chromium or the like.
According to the photovoltaic device of the above described construction, the incident light is mainly absorbed by the intrinsic a - Si layer (i), and electrons and holes are generated in the intrinsic a - Si layer (i) and are respectively taken out of the metal electrode portion 3 and the transparent conductive film 2 by an internal electric field formed by the p-type a - Si layer (p) and the n-type a - Si layer (n).
In this type of photovoltaic device, therefore, the optical properties and particularly, the optical band gap of the intrinsic a - Si layer mainly determines a wavelength region of light which can be absorbed. Accordingly control of the optical properties of the a - Si film in such a portion is important.
Therefore, as the amorphous semiconductor material in the intrinsic portion, an a - SiGe film having a narrow optical band gap is used so as to absorb more light in a long wavelength region.
FIG. 8 is a diagram showing a device structure of another prior art photovoltaic device. Although the above described photovoltaic device shown in FIG. 7 absorbs light only by one intrinsic amorphous semiconductor, the photovoltaic device shown in FIG. 8 uses two intrinsic amorphous semiconductors and the intrinsic amorphous semiconductors on the side of light incidence (il) and on the side of light transmission (i2) are respectively constituted by an a - Si film and an a - SiGe film so as to make the respective light absorption characteristics of the intrinsic amorphous semiconductors different from each other. In FIG. 8, the same portions as those shown in FIG. 7 are assigned the same reference numerals.
By the above described construction of FIG. 8, light incident from a substrate is first absorbed by a first light generating portion (SC1), and light which is not absorbed by the light generating portion (SC1) is absorbed by a second light generating portion (SC2). Particularly, the second light generating portion (SC2) is constituted by an a - SiGe film having a narrow optical band gap, thereby to make it possible to sufficiently absorb light in a long wavelength region which cannot be absorbed by the first light generating portion (SC1).
Such a photovoltaic device constructed by layering a plurality of light generating portions (SC) is generally referred to as a multi-junction solar cell, which is described in detail in Japanese Patent Publication No. 48197/1988, for example.
As described in the foregoing, the amorphous semiconductor material has the advantage in that the optical band gap can be changed into various values by doping an element such as germanium (Ge), nitrogen (N), carbon (C) or tin (Sn) into the a - Si film, while having the disadvantage in that the quality of the film is degraded by doping the element, as compared with the quality of the a - Si film into which no element is doped.
Particularly in an a - Si series material, it has been found that the content of hydrogen in the film and its bonding configuration exert an important effect on film properties. In the a - SiGe film into which Ge is doped, however, the doped Ge hydrogen in the film are subtly each other, whereby control of the element becomes complicated.
The fact that the probability of bonding between hydrogen and Si is larger by approximately ten times than the probability of bonding between hydrogen and Ge in the a - SiGe film containing not more than 50% Ge is considered the cause.
In order to solve such a problem, therefore, a method of alternately forming an a - SiGe film and performing hydrogen plasma processing to improve the film quality has been tried (see Japanese Patent Laid-Open Gazette No. 84512/1987).
In the conventional method, however, it cannot be desired to rapidly improve the film quality for the decrease in productivity, and it is difficult to control the behavior of hydrogen in the film, whereby the fundamental solution has not been achieved.